1. Field of the Invention
The present invention is directed generally toward data storage and backup systems. More specifically, the present invention is directed toward a backup system that immediately backs up data written to a primary storage device to multiple “virtual mirrors,” that reflect the changing state of the primary storage device over time.
2. Background of the Invention
A data replication system can provide an up-to-the-minute duplicate copy or replica of changing data on a storage device. Write commands issued to a primary storage device are duplicated and issued to the data replication system, which records the written data in its own storage medium. The simplest form of data replication is a redundant “mirror” volume, which is sophisticated data replication systems store not only a current duplicate copy of the primary device but also allow additional past-time images of the primary device to be accessed. This may be done through some kind of “journaling,” where the write commands themselves are archived, rather than simply a copy of the data.
Sometimes, however, communication to the data replication system is lost. This may be for a variety of reasons. For example, a physical connection with the device hosting the data replication system may be broken, or communication software may malfunction. When this happens, a data replication system will be out of synchronization with the primary storage device. Some reconciliation process is necessary to restore synchronization between the data replication system and the primary storage device.
Performing this reconciliation well is not a trivial task. One easy way to reconcile a primary storage device with its data replication system is simply to temporarily take the primary storage device out of service and copy the contents of the primary storage device to the data replication system. It is an undesirable technique, as it requires taking the primary storage out of service.
Another way to reconcile the two storage systems is to temporarily (while the data replication system is unavailable) store the duplicated write commands in an auxiliary journal, then “replay” the write commands for the data replication system when it comes back up. This is also undesirable, since it requires extra storage for the auxiliary journal.
What is needed, then, is a mechanism for the resumption of data replication that is minimally intrusive and that requires a minimal amount of extra storage space.